A hydraulic drive system, which is sometimes called a load sensing system, controls the delivery flow rate of a hydraulic pump so that the delivery pressure of the hydraulic pump (main pump) is higher than the maximum load pressure of a plurality of actuators by a target differential pressure. Such a load sensing system is configured such that differential pressures across a plurality of flow control valves are each kept at a given differential pressure by a pressure compensating valve so that a hydraulic fluid can be fed to the plurality of actuators at a ratio depending on opening areas of the flow control valves regardless of the load pressures during the combined operation in which the actuators are simultaneously driven.
The load sensing system described above exercises control as follows: a differential pressure (hereinafter, called the differential pressure PLS) between a delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators is led to pressure compensating valves; a target compensating differential pressure of each pressure compensating valve is set based on the differential pressure PLS; and a differential pressure across the flow control valve is kept at the differential pressure PLS. During the combined operation where the plurality of actuators are simultaneously driven, a saturation state where the delivery flow rate of the hydraulic pump is insufficient may occur. In such a state, the differential pressure PLS is lowered depending on the degree of the saturation and the target compensating differential pressure of the pressure compensating valve, i.e., the differential pressure across the flow control valve is reduced. Thus, the delivery rate of the hydraulic pump can be redistributed at a ratio of flow rates demanded by the respective actuators.
In patent document 1, the load sensing system described above is provided with a differential pressure reducing valve which outputs, as an absolute pressure, the differential pressure PLS between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators. The output pressure of the differential pressure reducing valve is led to the plurality of pressure compensating valves to set respective target compensating differential pressures. The load sensing system is provided with a differential pressure reducing valve which outputs, as an absolute pressure, the pressure according to the revolution speed of an engine driving the hydraulic pump. The output pressure of this differential pressure reducing valve is led to a load sensing control regulator and the target differential pressure of the load sensing control is set as a variable value according to the revolution speed of the engine.